Field of the Disclosure
This invention is related to image sensors. In particular, embodiments of the present invention are related to three dimensional image sensors.
Background
Interest in three dimensional (3D) cameras is increasing as the popularity of 3D applications continues to grow in applications such as imaging, movies, games, computers, user interfaces, and the like. A typical passive way to create 3D images is to use multiple cameras to capture stereo or multiple images. Using the stereo images, objects in the images can be triangulated to create the 3D image. One disadvantage with this triangulation technique is that it is difficult to create 3D images using small devices because there must be a minimum separation distance between each camera in order to create the three dimensional images. In addition, this technique is complex and therefore requires significant computer processing power in order to create the 3D images in real time.
For applications that require the acquisition of 3D images in real time, active depth imaging systems based on the optical time of flight measurement are sometimes utilized. Time of flight systems typically employ a light source that directs light at an object, a sensor that detects the light that is reflected from the object, and a processing unit that calculates the distance to the object based on the round trip time that it takes for light to travel to and from an object. In typical time of flight sensors, photodiodes are often used because of the high transfer efficiency from the photo detection regions to the sensing nodes. Separate circuitry is coupled to the photodiodes in each pixel cell to detect and measure the light that is reflected from the object.
However, a continuing challenge with the acquisition of 3D images using time of flight systems is that there are pixel-by-pixel variations in the separate circuitry coupled to the photodiodes in each pixel cell. For instance, it is not unusual for there to be approximately 5% differences between pixel cell current mirror outputs across a time of flight sensor due to pixel-by-pixel variations that may occur in the sensors. These pixel-by-pixel variations in current mirror outputs consequently reduce the accuracy and reliability of time of flight sensors.
Corresponding reference characters indicate corresponding components throughout the several views of the drawings. Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of various embodiments of the present invention. Also, common but well-understood elements that are useful or necessary in a commercially feasible embodiment are often not depicted in order to facilitate a less obstructed view of these various embodiments of the present invention.